Vidicon target consisting of silicon dioxide layer on silicon

ABSTRACT

The present invention relates to targets for VIDICON tubes comprising monocrystalline semiconductors. The target comprises a uniform monocrystalline semiconductor layer with N-type doping, (17), having a narrow forbidden band upon which a transparent metal layer (18) is deposited which is exposed to the incident light radiation. A dielectric layer (19) is deposited upon the layer (17) in order to receive the impact of the electron beam.

0 United States Patent 1 91 1111 3,890,523 Poirier June 17, 1975 [541VlDICON TARGET CONSISTING OF 3,403,284 9/1968 Buck t a1 315/11 SILICONDIOXIDE LAYER ON SILICON 3,523,208 1 8/1970 Bodmer et a1. 315/103,677,833 7/1972 Le Coq 313/65 AB [75] Inventor: Raymond Poirier, Pans,France .[73] Assignee: Thomson-CSF, Paris, France primary ExaminerRobert Sega] [22] Filed: 13 1972 ttofiney, Agent, or Firm-Cushman, Darby&

us man [21] Appl. No.: 314,619 Related US. Application Data T [63]Continuation of Ser. No. 130,625. April 2. 1971, [57] ABS RACTabandoned" The present invention relates to targets for VIDICON F A r rP 1 D t tubes comprising monocrystalline semiconductors.

A $1 l y a a 70 l 254] The target comprises a uniform monocrystallinepr. rance Semiconductor layer with yp p g (17), having a narrowforbidden band upon which a transparent 2% g metal layer (18) isdeposited which is exposed to the J313/65AB C 66 incident lightradiation. A dielectric layer (19) is I 1 o earc deposited upon thelayer (17) in order to receive the References Cited impact of theelectron beam.

UNITED STATES PATEN 3 Claims, 4 Drawing Figures 3,403,278 9/1968 Kahnget al. 313/65 AB PATENTEDJUN17 i975 3,890,523

SHEET 1 I I I VIDICON TARGET CONSISTING OF SILICON DIOXIDE LAYER ONSILICON This is a continuation of application Ser. No. 130,625, filedApr. 2, 1971, now abandoned.

The present invention relates to a target comprising a monocrystallinesemiconductor, designed for electronic tubes of the vidicon type, forthe television camera applications.

Known targets of this type are in the form of mosaics of photo-diodes,each with a pn-junction.

These targets are complex and, moreover, it is difficult to manufacturea pn-junction with a semiconductor having a narrow forbidden band.However, semiconductors of this type are the only ones which give avidicon tube a good spectral response within the long wave range, thatis to say the infra-red.

According to the invention, threre is provided a target for cathode-raytubes intended for producing a video signal, comprising an n-dopedsemiconductor layer having a face exposed to the incident lightradiation, and an other face and deposited on said other face, adielectric layer upon said semiconductor layer in order to take theimpace of the electron beam;

the same conductor and the dielectric layers having an interface, havingfree energy levels in the forbidden band of the semiconductor, fortrapping positive charge carriers and means for applying to said face ofsaid semiconductor layer a predetermined d.c. potentiel.

The invention will be better understood from a consideration of theensuing description and by reference to the attached drawings in which:

FIG. 1 schematically illustrates a VIDICON tube.

FIG. 2 illustrates a schematic transverse section through the target inaccordance with the invention.

FIG. 3 illustrates the electrical charges in the target of FIG. 2.

FIG. 4 is the target energy diagram, in accordance with the transversesection of FIG. 2.

FIG. 1 illustrates the essential elements of a conventional VIDICONtube. The electron gun 1 comprises a source of electrons whichsuccessively scan the target 3 through the meshes of the grid 4, bydeflection of the electron beam 2. The symbols 5, 6 and 7 respectivelydesignate the collimating coil, the focusing coil and the deflectionunit. The objection or scene being reproduced, 8, transmits the lightrays 9 onto the lens 10, the latter forming an image of said object orscene upon the target, through the glass wall 11 of the tube.

The operation of the tube is well known.

The video signals in particular, are picked up at the output 12.

These signals are transmitted by the resistancecapacitance arrangement(13-14) to the video output 15.

The output 12 is at a positive potential V; in relation to that of thecathode. The corresponding voltage source 16 is connected to the output12 across the resistor 13.

An embodiment of the target according to the invention is shown in FIG.2. It comprises an N-type semiconductor layer 17, for example silicon,upon which a transparent conductive layer 18 has been produced, forexample in the form of a deposit of tin oxide having a thickness of somefew hundreds of A. The layer 18 is exposed to the incident lightradiation. A dielectric layer .19 is"deposited upon the layer 17 inorder to receive. the .impactof theelectron beam from the cathode. It isconstitutedyfor example by a silicon oxide layer"formedbyi anodicoxidation on the silicon semiconductorlayerwhichispreferably doped at alevel of around 10 to. 1 01iatms/cm.

The operation-of the target is as follows:

The electron beam coming from the electron gun produces upon the exposedsurface of the layer 19, negative charges 20 represented by the sign inFIG. 3. The charges 20 induce in the semiconductor 17 a positive spacecharge 21 constituted by ionised impurity atoms. These ions 22 arefixed. In a general way, the charges 20, under the influence of theelectric field, created by the difference of potentiel between source 16and cathode 1 pass with a greater or lesser degree of difficulty throughthe layer 19. In the following paragraph the conditions governing thistransfer will be explained. In the dark condition, the charges 20 arearrested by the space charge and cannot reach the layer 18. Underilluminated conditions, the photons of higher energy than the forbiddenband 23 shown in FIG. 4, will be absorbed and will create electronholepairs. In the energy diagram of FIG. 4, the abscissae OX plot thedistances of points on the target from a plane parallel to the frontface of the dielectric layer 19 and located upstream thereof in relationto the electron beam originating from the cathode. The ordinates OE,plot the energies or the potentials of changed sign. The charge carriersthus created, shown in FIG. 4 by the signs and will be subjected to theelectric field due to the difference of potential between V and thepotential of the cathode. The electrons will diffuse towards the layer18, remaining in the conduction band 24. The holes will diffuse towardsthe dielectric, remaining in the valence band 25. When they arrive atthe interface between semiconductor and dielectric, they may either beneutralised by electrons which have passed through the dielectric, ormay accumulate there. In the latter case, they will occupy free energylevels in the forbidden band and will be trapped there at 26. Accordingto the invention, the semiconductor and the dielectric are chosen insuch a manner that these energy levels exist at the interface.

It is because of this phenomenon that the lateral conductivity is low atthe interface. The lateral conductivity is low at the surface of thedielectric layer submitted to the electron bombardment. Thus, theresolving power is improved.

We will now see how the electrons can pass through the dielectric whichhas been assumed to be relatively thick for example in order of 500 A.

The free charge carrier movement in the semiconductor layer allows adecreasing of the space charge. The resistivity of the semiconductordecreases, and the potential difference between the interface and theface of the layer exposed to the light decreases. The electric field inthe dielectric increases, and also the leakage current in thedielectric. Thus, the electric charges deposited by the electron beam ateach scanning tune, flow across the structure. The electric current dueto this movement is a direct function of the light beam intensity.

A second possible mode of conduction which arises in the case of adielectric layer having a thickness in the order of 50 A, in factsilicon oxide at the surface of the doped silicon, will now bedescribed. The dielectric is traversed by a tunnel effect mechanism. Theconductivity then depends upon the number of cases available forelectrons at the surface of the semiconductor, that is to say upon thenumber of free "or trapped holes at the interface and consequentlyupon-the illumination.

Amongst the advantages of theinvention it will be observed that thetarget is constituted by a single type (N) of semiconductor, there beingno necessity to produce a pn-junction within its thickness.

In addition, the semiconductor layer is uniform, that is to say it isnot necessary to produce a mosaic of elements.

Finally, by using a semiconductor material with a narrow forbidden band,this being made possible a good spectral response to visible light andinfra-red radiation, is obtained.

The invention can be applied to any cathode ray tube in which it isdesired to reproduce an image formed on the target.

What I claim is:

1. In a cathode ray tube intended for producing a 4 video signal; atarget consisting of:

an N-type silicon semiconductor layer having a face exposed to theincident light radiation and another v face directed towards saidcathode and deposited on'sai diother face, a silicon dioxide dielectriclayer upon said semiconductor layer having a uniform thickness comprisedbetween 50 and 500 A in order to take the impact of the electron beam,the semiconductor and the dielectric layer having an interface, havingfree energy levels in the forbidden band of the semiconductor fortrapping positive charge carriers, and means for applying to said faceof said semiconductor layer a predetermined d.c. potential.

2. A target as claimed in claim 1, characterised in that said dielectriclayer has a thickness in the order of 50 A.

3. A target as claimed in claim 1, wherein the dielectric layerthickness is of the order of 500 angstroms.

* =l= =l l

1. IN A CATHODE RAY TUBR INTENDED FOR PRODUCING A VIDEO SIGNAL, A TARGETCONSISTING OF: AN N-TYPE SILICON SEMICONDUCTOR LAYER HAVING A FACEEXPOSED TO THE INCIDENT LIGHT RADIATION AND ANOTHER FACE DIRECTEDTOWARDS SAID CATHODE AND DISPOSITED ON SAID OTHER FACE, A SILICONDIOXIDE DIELECTRIC LAYER UPON SAID SEMICONDUCTOR LAYER HAVING A UNIFORMTHICKNESS COMPRISED BETWEEN 50 AND 500 A IN ORDER TO TAKE THE IMPACT OFTHE ELECTRODE BEAM, THE SEMICONDUCTOR AND THE DIELECTRIC LAYER HAVING ANINTERFACE, HAVING FREE ENERGY LEVELS IN THE FORBIDDEN BAND OF THESEMICONDUCTOR FOR TRAPPING
 2. A target as claimed in claim 1,characterised in that said dielectric layer has a thickness in the orderof 50 A.
 3. A target as claimed in claim 1, wherein the dielectric layerthickness is of the order of 500 angstroms.